For instance, I used Arlen Ness turn signals, but rather than bolting them into the standard set up, drilled holes and created fiberglass shims for them so that they could be fitted directly into the rear fender without being crooked (fender angles slightly outward when viewed from above). For the fronts, I actually drilled holes in the casing and used the wiring from the stock turn signals, rather than the hollow bolt which came with the new ones. I don't know that it necessarily would be considered custom work, but its not really just bolting stuff on anymore either.
Also, What is a one-off part? Like one of a kind?
I don't consider my bike to be a "custom" bike, but am interested in what others consider that to mean for them.
